1. Field of the Invention
This invention is directed to a low density polyethylene having broad molecular weight distribution and substantially uniform branching frequency, and to a method of preparation thereof. More particularly, the invention is directed to a low density polymer having a broad molecular weight distribution and excellent strength properties, but relatively low level of hexane extractables.
2. Description of Related Art
High density polyethylene (HDPE) polymers having broad molecular weight distribution and high molecular weight have been proposed in the past. Such polymers are usually made by blending high molecular weight, high density component with a low molecular weight, high density component. The high molecular weight component has low melt index (e.g., I.sub.21 of about 0.1 to 1.0 g/10 min), while the low molecular weight component has a high melt index (e.g., I.sub.2 of at least about 100 g/10 min). Both the low molecular weight and the high molecular weight components have substantially the same densities of at least about 0.940 g/cc, indicating that the melt index-corrected density, and therefore the branching frequency, of the two component resins differ greatly from each other.
These blends have good physical properties, derived from the high molecular weight component, and good processability, provided by the low molecular weight component. However, such a method of producing the blends is restricted to high density polymers because it is believed in the art that if low density, broad molecular weight distribution polymers were made by blending a high molecular weight, low density component with a low molecular weight, low density component, the resulting blend would produce end use products, such as films, having high hexane extractables. As is known to those skilled in the art, high hexane extractables are undesirable because films and other articles made from such polymers may not meet strict Food and Drug Administration (FDA) requirements which limit the amount of hexane extractables. Additionally, high levels of hexane extractables may cause operational difficulties, e.g., in extruders the extractable material tends to separate out in the die and drip therefrom.
It is also known that low density and medium density copolymers of ethylene with higher alpha-olefins, such as 1-hexene or 1-octene, have good strength properties, e.g., good tear strength resistance. However, such polymers have relatively narrow molecular weight distribution which is undesirable in some applications, e.g., wherein broad molecular weight distribution is needed to exploit the benefits of the high molecular weight fraction, such as increased film strength.
Polymers having multi-modal molecular weight distribution are also known in the art. Multi-modal molecular weight distribution means that the molecular weight distribution of the polymer is relatively broad and two or more peaks of different molecular weights are readily discernible in a plot of molecular weight as a function of relative proportions of the polymer having the given molecular weight, such as that obtained by gel phase chromotography (GPC) analysis of the polymer, e.g., see Morita et al, U.S. Pat. No. 4,338,424.
Bailey et al, European Published Patent Application 0 100 843, published on Feb. 2, 1984 disclose blends of two high density polymers: a high molecular weight polymer and a low molecular weight polymer, both preferably having narrow molecular weight distribution and low levels of long chain branching. The blends produce films having good properties and environmental stress crack behavior.
It is the primary object of the present invention to provide a low density polymer, particularly linear low density polyethylene (LLDPE), having relatively broad molecular weight distribution, and therefore good processibility at high molecular weights. This combination of low density and broad molecular weight distribution provides exceptional impact strength, tear resistance, and relatively low levels of hexane extractables.
Other objects of the invention will become apparent to those skilled in the art from the following description thereof.